Known projectors often accomplish image projection by passing light through a panel which has an image on it, or by reflecting light off a panel which has an image on it. The image may be formed in photographic emulsion. Alternatively, the image may be formed by controlling the polarisation of pixel elements such that light is passed or absorbed according to the intensity required to form the image. A further alternative involves the use of moveable micro-mirrors or oil films that may be used to direct light to an aperture or absorbing light dump. In some of these known devices, the panel is able to modulate all colours so that a coloured image may be formed. In others of the known devices, three panels are used to modulate red, blue and green light separately. A further method is for one panel or two panels to modulate the three colours in rapid sequence, this being known as field sequential colour.
The known projectors comprise a light source, collector means for collecting the light and focusing it into a beam, and an optical train which directs the light to the panel. In some cases, the light is split into three colours on the way. The light is transmitted through or reflected from the panel, and then it exits the illumination apparatus through a lens system which is designed to cast the image onto a screen. The transmission of light in this manner is generally fairly inefficient. More specifically, light beams can be considered as a collection of rays propagating at a range of angles rather than a bunch of parallel rays. If optical components are used to widen or narrow a beam, it is found that the product of the solid angle subtended by the rays in the beam and the cross-sectional area of the beam remains constant. Any aberration or inefficiency in the optical components has the effect of increasing this value, which is known as etendue. The etendue of a beam of light can only be decreased at the expense of energy in the beam,
In any light path, there may be one optical component whose etendue is the smallest, thus limiting the maximum amount of light that can propagate through the apparatus. It is preferable therefore to use a light source with a minimum etendue, as then the entire optical beam can be coupled into a small area of high f-number. Such a source would have a volume, which was as small as possible. This is why arc lamps are currently the most popular source of illumination for projectors.
Some kinds of light source including light emitting diodes and arc lamps but not filament lamps have the ability to produce light at a much higher intensity when operated for a short duration, compared with their maximum continuous output. When operated in flashing mode at a given mark space ratio, the average light output may be the same as or less than the continuous output but the instantaneous output during the flash is high.
It is an aim of the present invention to provide improved illumination apparatus for example for projection.